A fuel cell is a device that converts chemical energy of a fuel into electricity. Fuel cells are attractive for a variety of reasons, e.g. high energy efficiency, low pollution, fuel flexibility, high quality power output, quick response to load fluctuations, excellent heat recovery characteristics, quiet operation, etc. Their high energy efficiency and low pollution are partially attributed to the use of a clean fuel source, e.g. hydrogen, methanol, etc. Fuel cells can be utilized in various contexts, e.g. as power sources for computers, camcorders, mobile phones, automobiles, trains, ships, submarines, heat and electric supply to homes, and electric power plants. Recent efforts have been directed to developing fuel cells as a power source in automobiles and mobile phones.
The structure of a basic fuel cell consists of two electrodes, i.e. an anode and a cathode, and an electrolyte. Oxygen passes over one electrode and hydrogen passes over the other electrode to generate electricity, water, and heat. In a catalyzed reaction, the hydrogen atom splits into a proton and an electron, which travel to the cathode along different paths. The proton passes through the electrolyte while the electrons create a separate current that can be utilized before returning to the cathode to be reunited with the hydrogen and oxygen in a molecule of water.
Fuel cells can be classified according to their operating temperatures, i.e. high-temperature type or low-temperature type, depending on the kind of electrolytes used. For instance, a low-temperature fuel cell has high catalytic activity and ion conductivity at relatively low temperatures for generating a desired amount of energy. As recognized by those of ordinary skill in the art, the catalyst and electrolytes employed in a fuel cell have significant impact in determining the functional specifications of a fuel cell.
Fuel cells that operate at relatively low temperatures, e.g. about 80° C. to about 100° C., generally utilize active catalysts made of a high-dispersion precious metal such as platinum. As such, the development of fuel cells that operate on minimal quantities of catalyst is crucial to achieving high performance and reliability at low cost. While an increase in the surface area of platinum catalyst particles can increase catalytic activity by enhancing the reactive surface area exposed to reactants, there are practical considerations which limit how this increase in surface area may be implemented. In membrane electrode assembly (“MEA”), increased thickness of the catalyst layer increases the internal resistance of MEA, which causes the output of the fuel cell to drop. A thick catalyst layer would slow the rate of gas diffusion which is critical to efficient fuel cell function.
In the face of these difficulties, the development of platinum supported catalysts with a high loading and dense dispersion of platinum particles of minimal size remains an ongoing endeavor. There is still a need in the art to develop catalysts having reduced requirements for platinum or at least optimized activity per unit weight of platinum without suffering a dramatic decrease in its energy efficiency.
Conventional methods for preparing Pt/support powder by loading Pt particles on a support are broadly classified into precipitation methods or colloidal methods. In precipitation methods, the procedure is carried out primarily in liquid state and relatively adaptable for mass production. However, many conventional precipitation methods tend to produce non-uniform dispersion of relatively large platinum particles.
In colloidal methods, fine platinum particles are synthesized in an aqueous solution or an organic solution and then adsorbed onto a carbon support. Colloidal methods require an extra hydrogen reduction step, are generally more time-consuming, and difficult to control with respect to the platinum particle size resulting therefrom. Slight variations in the reaction conditions, e.g. pH or temperature or duration of such conditions, can have tremendous impact on the particle size. For example, in the manufacture of platinum colloids using organic materials such as ethylene glycol, non-uniformly sized platinum colloids will result unless the temperature is increased to the reducing temperature, i.e. about 150° C., within 20 minutes. While it is possible to produce fine platinum colloidal particles at basic pH in an aqueous environment, the process involves more time and/or higher temperature conditions for reduction to occur. As such, there is a need in the art for improved methods for preparing high-dispersion supported platinum catalyst.